For maximum i/o pins, then the Arduino/Genuino (outside US name) Mega is the best. Upto 54 digital i/o pins and upto 16 analogue inputs. It's not the cheapest to get started with, but it all depends on what you want to do with it. Clones are available, but I've had some cheap ones from China which don't do all they should. Getting them from a UK source seems to be the best middle road option.

Although not as capable as a Pi, they work as soon as power is supplied and are much less power hungry. There are always alternatives to anything, but the arduino user base is huge as is the number of webpages devoted to them..